As a typical structure of lithium ion secondary batteries, there has been known a so-called “wound structure” in which an electrode assembly formed by combining a positive electrode plate, a negative electrode plate, and a separator is spirally wound. Among them, lithium ion secondary batteries in which such an electrode assembly is wound into a flat shape are much in demand in the field of mobile devices, etc. In order to prevent the wound electrode assembly from being loosened, the winding end portion of the electrode assembly is usually fixed with an adhesive member (typically an adhesive tape).
Meanwhile, a phenomenon that an electrode assembly expands as a battery is charged is known to those skilled in the field of lithium ion secondary batteries. Specifically, a positive electrode plate expands in volume as a positive electrode active material is dedoped with lithium ions, and a negative electrode plate expands in volume as a negative electrode active material is doped with lithium ions. However, since the winding end portion is fixed with an adhesive member, the outward expansion of the electrode assembly is restricted. Therefore, a relatively large stress is generated in the electrode assembly, and the electrode assembly may be deformed to release this stress. For example, the electrode assembly is deformed like folded strata in some cases. The deformation of the electrode assembly often accompanies an increase in the thickness of the battery.
In order to address this problem, JP 2006-302801 A describes a technique for fixing the winding end portion with an adhesive member provided with a folded portion in a part of the substrate where no adhesive is present. With the use of the adhesive member described in this patent literature, the folded portion is unfolded and thereby allows the winding of the electrode assembly to be loosened. This means that a space is secured to absorb the expansion of the electrode assembly even after it is wound. This can suppress the increase in the thickness of the battery during charging.